1. Field of the Invention
The present invention relates to a dual slope analog-to-digital converter with automatic, short cycle range determination and, more particularly, to a simple and inexpensive dual slope analog-to-digital converter having a minimum number of components.
2. Description of the Prior Art
Integrating analog-to-digital converters have two characteristics in common. First, their output represents the integral or average of an input voltage over a fixed period of time. Thus, an integrating converter will give repeatable results in the presence of high frequency (relative to the measurement period) noise. Secondly, they use time to quantize the answer resulting in extremely small non-linearity errors and no possibility of missing output codes. Furthermore, the integrating analog-to-digital converter has very good rejection of frequencies whose periods are an integral multiple of the measurement period. This feature can be used to advantage in reducing line frequency noise. The unique characteristics of the integrating converter have made it the natural choice for panel meters and digital voltmeter applications.
The most popular integrating converter is the "dual slope" type. In such a converter, conversion takes place in three distinct phases. The first phase is an autozero phase during which the errors in the analog components will be automatically nulled out by grounding the input and closing a feedback loop such that error information is stored on an auto-zero capacitor.
The second phase is the signal integrate phase during which an integrator capacitor is charged with an unknown current, proportional to an unknown voltage V.sub.IN, for a fixed period of time, typically for a fixed number of clock pulses. For a 31/2 digit converter, 1,000 pulses is the usual count. For a 41/2 digit converter, 10,000 pulses is typical. On completion of the integration period, the voltage V on the integrator capacitor is directly proportional to the input signal.
The third phase is the deintegrate phase during which the integrator capacitor is discharged with a known current for a variable period of time. More specifically, at the beginning of the deintegrate phase, the integrator input is switched from V.sub.IN to a reference voltage V.sub.REF. The polarity of the reference voltage is determined during the integrate phase such that the capacitor discharges back toward zero. Clock pulses are counted between the beginning of the deintegrate phase and the time when the integrator output passes through zero. The number of clock pulses counted is a digital measure of the magnitude of V.sub.IN.
The advantage of a dual slope analog-to-digital converter is that the theoretical accuracy depends only on the absolute value of the reference voltage and the equality of the individual clock pulses within a given conversion cycle. The latter can easily be held to one part in 10.sup.6, so in practical terms, the only critical component is the reference voltage. Changes in the value of other components, such as the integration capacitor or the comparator input offset voltage have no effect, provided they don't change during an individual conversion cycle.
In spite of the above, several problems are encountered when one desires to develop a very inexpensive digital multimeter and one having several ranges of operation. The only way to bring cost down is to replace as many switches as possible and eliminate costly precision components and to more fully integrate. That is, in the past, in order to handle a number of different ranges, a number of resistor-divider networks have been used at the input of a multimeter. These resistor-dividers must be precision resistors, resulting in a high cost and the resistors cannot be included in an integrated circuit.
Another alternative is to change ranges by changing the time during the integrate phase. With such a technique, the instrument can automatically change ranges until the correct range is found. However, all previous autoranging instruments require the full integrate/deintegrate time period to make a measurement and determine whether or not the input is within the desired range. If the instrument is on the wrong scale, it will continue indicating "over range" until it changes to the correct range. This is quite undesirable because it is time consuming and an annoyance to the user of the instrument.